Electro-optic Tuning Research Articles

Electrically controllable diffractive optical elements fabricated by direct laser writing on a carbon nanotube network film

Abstract A randomly connected single-walled carbon nanotube (CNT) network film is suggested as an optically homogenous thin film to implement a tunable diffractive optical element with a subwavelength thickness. A Fresnel zone plate (FZP) as a thin-film lens is successfully realized by mask-free direct laser writing onto the CNT network film with a thickness of 450 nm. The fabricated FZP exhibits an intense three-dimensional focus having lateral and axial focal sizes of 0.95λ and 7.10λ, respectively, at the wavelength of 1550 nm. Furthermore, we show that the intensities at focal points of the first and second diffraction orders can be significantly modulated by 72% and 40% through ion-gel gating between +1.8 V and −1.8 V. These results may offer the potential for electro-optic tunability in multifocal diffraction flat optics and the like.

Electro-optically tunable cascaded second-order nonlinearity induced (2 + 1)D spatial optical soliton using bulk β-barium borate crystal: a theoretical approach

This paper deals with the development of an effective theoretical approach to generate spatial optical soliton in exceptionally beneficial bulk beta-barium-borate crystal by utilizing cascaded second-order nonlinearity through electro-optic tuning. The proposed methodology paves the way for novel nonlinear optical interactions conceptualized in bulk material depending upon an effective nonlinear refractive index for a broad range of crystal lengths. The development of ( 2 + 1 ) D spatial bright soliton has been analytically demonstrated at 800 nm wavelength of the incident continuous-wave optical radiation, and the implication of tunable electro-optic effect to the crystal’s nonlinear response is also presented here. Involvement of the generalized nonlinear Schrödinger equation has been utilized to develop the necessary and sufficient criteria for self-trapping optical beams considering both linear and nonlinear absorption losses. In conformity with the findings of the simulation, the beam diameter of a generated spatial soliton within this anisotropic crystal is near 56 µm while maintaining precise peak intensity ∼ 0.8 G W / c m 2 .

Ultrafast terahertz transparency boosting in graphene meta-cavities

Abstract As an exceptional nonlinear material, graphene offers versatile appealing properties, such as electro-optic tunability and high electromagnetic field confinement in the terahertz regime, spurring advance in ultrashort pulse formation, photodetectors and plasmonic emission. However, limited by atomic thickness, weak light–matter interaction still limits the development of integrated optical devices based on graphene. Here, an exquisitely designed meta-cavities combined with patterned graphene is used to overcome this challenge and promote THz-graphene interaction via terahertz location oscillation. By using an 800 nm pump laser, the local field-induced strong interaction allows sensitive responses to the ultrafast energy transfer from the ultrafast optical pump to graphene electron heat, enabling 46.2% enhancement of terahertz transparency. Such optical modulation of terahertz waves shows ultrafast response in delay less than 10 ps. Moreover, thanks to the nature of graphene, the device shows unique potential for electrically dynamic tuning and further bandwidth broadening.

Electro-Optical Modulation in High Q Metasurface Enhanced with Liquid Crystal Integration.

Electro-optical tuning metasurfaces are particularly attractive since they open up routes for dynamic reconfiguration. The electro-optic (EO) modulation strength essentially depends on the sensitivity to the EO-induced refractive index changes. In this paper, lithium niobate (LiNbO3) metasurfaces integrated with liquid crystals (LCs) are theoretically investigated. Cylinder arrays are proposed to support quasi-bound states in the continuum (quasi-BICs). The quasi-BIC resonances can significantly enhance the lifetime of photons and the local field, contributing to the EO-refractive index changes. By integrating metasurfaces with LCs, the combined influence of the LC reorientation and the Pockels electro-optic effect of LiNbO3 is leveraged to tune the transmitted wavelength and phase spectrum around the quasi-BIC wavelength, resulting in an outstanding tuning sensitivity up to Δλ/ΔV ≈ 0.6 nm/V and relieving the need of high voltage. Furthermore, the proposed structure can alleviate the negative influence of sidewall tilt on device performance. The results presented in this work can foster wide application and prospects for the implementation of tunable displays, light detection and ranging (LiDAR), and spatial light modulators (SLMs).

All optical logic gates function by ring resonator properties aiding photonic crystal

Optical logic gates based on the Pockels effect are key components of light-wave communication networks and quantum computing because they are highly efficient and ultrafast. In this paper, we proposed a new hybrid platform of gallium arsenide and barium titanate (GaAs-BTO) for Ultraefficient Electro-Optic Tuning based on two-dimensional photonic crystals, the only proposed multifunctional structure is used to realize various very high-performance photonic logic gates such as BUFFER, NOT, AND, NAND, NOR, OR, XOR, XNOR. The functional parameters of these miniature logic gates are analyzed and optimized numerically by the FDTD method. The simulation results show that the contrast ratio is very high, with a very small footprint of 157 μm2, the response time is ultrafast 1 ps which corresponds to a bit rate of 1 Tbps.

Integratable electro-optic modulator based on a polymer-embedded silicon racetrack resonator with high electro-optic wavelength tuning.

Electro-optic (EO) modulators based on polymer-embedded silicon racetrack resonators (EOM-PSRR) are investigated. To obtain the single-mode propagation condition, the mode and transmission characteristics of the polymer-embedded silicon waveguide are simulated by the finite element method (FEM). By adding a static bias voltage, the EO modulation performances of EOM-PSRR embedded with lithium niobate (LiNbO3), EO polymer (AJ309), and hybrid EO polymer/TiO2 material (HEOT) are studied. The results show that the EOM-PSRR embedded with LiNbO3 achieves a high modulation depth (MD) of ∼27.6dB with a low EO wavelength tuning (λEO) of 10 pm/V. However, the EOM-PSRR embedded with HEOT has a high λEO of 100 pm/V but a low MD of ∼6.2dB with an extinction ratio of ∼5.2dB. The EOM-PSRR has potential application prospects in optical communication, optical signal processing, and optical network links. It can be produced as an optical frequency comb generator in a dense wavelength division multiplexing system, an EO frequency shifter for laser beams, an optical soliton former, and a photon time-delay device in a phased array radar.

Electro-optic tuning of a single-frequency ultranarrow linewidth microdisk laser

Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication, light detection and ranging, to metrology and sensing. Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality (Q) factor and supports only a single lasing frequency simultaneously. Here, we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate (LN) microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart, while their Q factors (above 10 million) are even higher due to the significantly reduced scattering on their propagation paths, single-frequency lasing with a linewidth as narrow as 322 Hz is observed. The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers. Finally, enabled by the strong linear electro-optic effect of LN, real-time electro-optical tuning of the microlaser with a high tuning efficiency of ∼50 pm / 100 V is demonstrated.

Cavity resonator-integrated guided-mode resonance filters with on-chip electro- and thermo-optic tuning.

Cavity resonator grating filters (CRIGFs) integrated on lithium niobate on insulator (LNOI) with electrical tuning elements are reported. The resonance wavelength of the filters is in the 780 nm range. Integrated thermo-optical tuning range of 2.5 nm is measured using integrated resistors, whilst a 0.7 nm electro-optical tuning range using capacitive metallic pads is achieved with ±400V drive voltage.

Electro-optic high-speed optical beam shifting based on a lithium niobate tapered waveguide.

We propose an electro-optic on-chip beam shifting device based on gradient microstructured electrodes and an optical tapered waveguide fabricated using lithium niobate (LN). The distribution of refractive index variations of the optical waveguide can be electro-optically defined and tailored by the designed gradient microstructured electrodes, which directs the beam propagation and shifting. The length of the beam shifting device is 18 mm and the width of the waveguide is gradually increased from 8 μm to 80 μm. The functionality of the beam shifting device is experimentally demonstrated, and it is observed that it has an electro-optic tunability of 0.41 μm/V, and a high-speed response time of 19 ns (λ=1310 nm). This study can provide potential applications in optical switching and modulation, beam scanning and ranging, optical spatial communications, etc.

Broadband spatial polarization processing of light via a photopatterned dichroic medium

Vector beams, featured by a specific space-variant distribution of polarization, have attracted intensive attention. Arbitrary and broadband spatial polarization processing is a key pursuit for vector beam generation, modulation, and detection. Here, we introduce black dye into liquid crystals to form a dichroic medium and imprint high-resolution patterns into the medium via photopatterning. Continuous cylindrical and binary vector beams are freely generated. Switching between a vector beam generator and a high-transmittance homeotropic state can be realized due to the electro-optical tunability of liquid crystals. On the basis of a radial vector beam generator array, the polarization distribution is characterized by calculating the darkest line of each generator. This strategy provides a platform for arbitrary spatial polarization processing and may bring more possibilities for polarization imaging, structured light generation, and materials characterization.

Wavefront shaping for reconfigurable beam steering in lithium niobate multimode waveguide.

Reconfigurable photonic devices are important constituents for future optical integrated circuits, where electro-optic manipulation of the light field in a lithium niobate (LN) waveguide is one of the promising solutions. Herein, we demonstrate a paradigm shift of the beam steering mechanism where reconfigurable beam steering is enabled by the wavefront shaping technology. Furthermore, this strategy is fully compatible with the electro-optic tuning mechanism of the LN multimode waveguide, where microstructured serrated array electrodes are employed to fine tune the output beam upon its reconfigurable output position. Our results provide new, to the best of our knowledge, insight for molding the flow of light in multimode waveguides and shed new light on beam steering photonic devices.

Quantum noise source based on shot noise of a balanced photodetector with a tunable integrated optical beam splitter

A broadband quantum noise source based on detection of shot noise of a balanced photodetector is demonstrated. Precise electro-optical tuning of the balanced photodetector circuit was carried out by an integrated optical beam splitter constructed in the form of the dual output Mach-Zehnder interferometer on a lithium niobate substrate. The classical component of the detected noise related to the relative intensity noise of a laser diode was suppressed by more than 15 dB. At the maximum laser power of 100 mW, the power spectral density of detected shot noise was 12 dB higher than the level of technical noise of the measuring system in the frequency band above 3 GHz. Keywords: quantum random number generator, vacuum fluctuations, shot noise measurements, integrated optics, lithium niobate

Design of gate-tunable graphene electro-optical reflectors based on an optical slot-antenna coupled cavity

The unique properties of graphene offer an exciting opportunity towards tunable photonic surfaces for flexible devices. In this paper, we design a gate-tunable, free-space graphene electro-optical reflector based on cavity resonator structures. We firstly calculate the graphene refractive index n and k as a function of Fermi level and external gating voltage. Then, we designed the structure of the single-layer graphene reflective resonator by carefully selecting suitable materials and device parameters to maximize the reflectance differences before and after electro-optical tuning. We also developed a theoretical model to discuss this system based on the optical transition matrix method. Moreover, we used field enhancement to further increase the reflectance differences by incorporating Sn nanodots based optical slot-antenna coupled cavities. The maximum broadband, incident angle insensitive reflectance differences could reach 28% with an extinction ratio of 1.62 dB at a low insertion loss of 0.45 dB, and the spectral range is tunable by changing the optical cavity length. We also used an indium tin oxide layer as part of the optical cavity and the electrode simultaneously to reduce the voltage applied. To our best knowledge, this work is the first one on tunable two-dimensional (2D) material reflectors for free-space applications, apart from using liquid crystals or magnetic metasurfaces. This new design of tunable 2D electro-optical reflectors also reduces the complexity of fabrication steps, having promising applications in tunable flexible photonic surfaces and devices for variable optical attenuators and light detection and ranging systems.

III/V-on-lithium niobate amplifiers and lasers

We demonstrate electrically pumped, heterogeneously integrated lasers on thin-film lithium niobate, featuring electro-optic wavelength tunability.

Ultracompact electro-optic waveguide modulator based on a graphene-covered λ/1000 plasmonic nanogap.

The extreme field confinement and electro-optic tunability of plasmons in graphene make it an ideal platform for compact waveguide modulators, with device footprints aggressively scaling orders of magnitude below the diffraction limit. The miniaturization of modulators based on graphene plasmon resonances is however inherently constrained by the plasmon wavelength, while their performance is bounded by material loss in graphene. In this report, we propose to overcome these limitations using a graphene-covered λ/1000 plasmonic nanogap waveguide that concentrates light on length scales more than an order of magnitude smaller than the graphene plasmon wavelength. The modulation mechanism relies on interference between the non-resonant background transmission and the transmission mediated by the gate-tunable nanogap mode, enabling modulation depths over 20 dB. Since the operation of the device does not rely on graphene plasmons, the switching behavior is robust against low graphene carrier mobility even under 1000 cm2/Vs, which is desirable for practical applications.

Optical edge imaging based on the birefringence property and electro-optic tunability characteristic of the liquid crystals

ABSTRACT Edge imaging is now an efficient way capable of demonstrating boundary features of an object, which is widely applied in optical imaging, robot vision. In this article, we propose an electrically regulated wedge liquid crystal (LC) cell for edge imaging based upon the combination of the large birefringence of LC and its electro-optic tunable characteristic, and its imaging principle of this element has been theoretically analysed. The experimental results wonderfully manifest that this method can achieve broadband edge imaging and realise real-time mode switch between edge image and original image which can be controlled by a proper voltage. We also demonstrate that the function for achieving electrically controllable resolution is feasible in theory.

Simulation and Analysis of Microring Electric Field Sensor Based on a Lithium Niobate-on-Insulator

With the increasing sensitivity and accuracy of contemporary high-performance electronic information systems to electromagnetic energy, they are also very vulnerable to be damaged by high-energy electromagnetic fields. In this work, an all-dielectric electromagnetic field sensor is proposed based on a microring resonator structure. The sensor is designed to work at 35 GHz RF field using a lithium niobate-on-insulator (LNOI) material system. The 2.5-D variational finite difference time domain (varFDTD) and finite difference eigenmode (FDE) methods are utilized to analyze the single-mode condition, bending loss, as well as the transmission loss to achieve optimized waveguide dimensions. In order to obtain higher sensitivity, the quality factor (Q-factor) of the microring resonator is optimized to be 106 with the total ring circumference of 3766.59 μm. The lithium niobate layer is adopted in z-cut direction to utilize TM mode in the proposed all-dielectric electric field sensor, and with the help of the periodically poled lithium niobate (PPLN) technology, the electro-optic (EO) tunability of the device is enhanced to 48 pm·μm/V.

Mid-infrared electro-optic modulator based on a graphene-embedded plasmonic rib waveguide with ultrahigh electro-optic wavelength tuning.

A graphene-embedded plasmonic rib waveguide (GEPRW) is designed for the mid-infrared electro-optic modulator. The mode characteristics and electro-optic (EO) modulation performances are simulated and optimized by using the finite element method. The results show that propagation length of 103mm and figure of merit of 106 are obtained by adjusting the bias voltage applied to the GEPRW. The EO wavelength tunings are -66.69 and -78.87nm/V for peak L and peak R in the loss spectra when w=3µm and h1=2µm. For a 100 µm long GEPRW, the modulation depths of ∼96.4,∼97.1,∼93.7, and ∼94.9%, and FWHMs of ∼30,∼74,∼34, and ∼59nm can be achieved when λ=1.55, 1.87. 1.89, and 2.23 µm. The EO modulator based on the GEPRW has a wide wavelength tuning range from 1.05 to 2.23 µm. It has high modulation depth, low insertion loss, and broad bandwidth, which can be used as EO tunable devices such as optical interconnects and optical switches.

Performance analysis of reconfigurable multifunctional photonic filters using triple coupled microring resonators

In this paper, we have introduced a new device architecture based on triple coupled microring resonators structure. The analytical model of the device is developed and performance analysis is carried out. The novelty of this device is that it performs as a multifunctional reconfigurable photonic filter in which multiple filtering functions such as notch, bandpass and bandstop responses are achieved by programming the coupling coefficients associated with it using thermal or electro optic tuning. Moreover, it can be used as a tuneable photonic true time delay line. As a bandpass reconfigurable filter, the maximally flat-top rectangular shape like bandpass response of 42 dB suppression ratio, and maximally flat-top bandwidth tunability of 1 GHz to 45 GHz are achieved. The notch filter response has an ultra-high rejection ratio of 148 dB. The flat bottom bandstop response has high suppression ratio of 81.5 dB within 7 GHz bandwidth. The device exhibits minimum group delay distortion over 18 GHz bandwidth, and a minimum of 8 ps and a maximum of 58 ps time delay tunability.

Electro-optic polymer ring resonator modulators [Invited

Electro-optic (EO) ring resonator modulators have a number of communications and scientific applications, including analog optical links, optical signal processing, and frequency comb generation. Among the EO materials used to fabricate ring modulators, the EO polymer has many promising characteristics, including a high EO coefficient of 100–200 pm/V (3–7 times larger than that of LiNbO3), an ultrafast EO response time (<10 fs), a low dielectric constant (3 to 4) with very little dispersion up to at least 250 GHz, and a straightforward spin-coating fabrication process. These inherent characteristics will be able to combine excellent EO properties with simple processing in achieving exceptional performance in a variety of high-speed optical modulation and sensing devices. This review focuses on the research and recent development of ring resonator modulators based on EO polymers. The first part describes the operation principle of EO ring resonator modulators, such as modulation mechanism, EO tunability, and 3 dB bandwidth. Subsequently, the emphasis is placed on the discussion of the ring modulators with EO polymers as the waveguide core and the improvement of EO modulation by using an EO polymer/titanium dioxide hybrid core. At the end, a series of EO polymers on silicon platforms including slot modulators, etching-free modulators, and athermal modulators are reviewed.